The bus later drove to a petitioner shelter in Fengtai district afterwards, Sun alleged.
Sun is one of 13 people who are appealing over a dispute involving 90 people and 18 million yuan, which has not been resolved despite a judgment in their favor eight years ago.
"We just hope journalists from CCTV can come down and report it, because today is National Legal Advocacy Day. Last night we heard over 100 people from Shenyang [Liaoning Province] and Shanghai were planning to demonstrate here, I don't know where they are now," Sun said.
Another of Sun's group, Liu Guijie, said that on Monday, they had passed a difficult night in a petitioner shelter.
"Beijing police took us on a bus from the Supreme People's Court [Dongcheng district] to Jiujingzhuang [in Fengtai district] last night. Over 100 people were in a 100-square-meter room and we weren't allowed out by men in uniform. I don't know if they were from the police," said Liu.
Wang Chunfa, director of Chaoyang district stability maintenance office, said he did not know of Jiujingzhuang.